dm zoned: ignore metadata zone in dmz_alloc_zone()

When looking up zones in dmz_alloc_zone() we need to ignore
metadata zones so as not to accidentally overwrite metadata.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Bob Liu <bob.liu@oracle.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>